The conventional industrial method of manufacturing ferrochrome or charge chrome is in the submerged arc electric furnace. Chrome ore, reducing agent and flux are fed continuously into the smelting furnace. Fine feed material makes furnace operation difficult and can lead to large chromium losses. Hence fine feed materials are either avoided or agglomerated before charging. Optionally agglomerates of ore and reducing agent can be preheated and/or prereduced before being fed to the electric furnace. Fine feed materials can be used if they are firstly agglomerated; for example by pelletizing or by high temperature fusing.
In the electric smelting furnace, energy is supplied through carbon electrodes immersed in the charge. Gases, resulting from the reaction of ore and carbon deep in the furnace, flow upward and are released at the top of the furnace charge.
Often the furnace tops are closed with a water-cooled cover which has openings for electrodes and charge delivery. The cover permits the collection of the gases generated. Much of this gas consists of carbon monoxide, which can then be used as a fuel. In some installations the furnace top is left uncovered and the gases are burned at the surface.
Accurate weighing and proportioning of the feed materials is essential for the successful operation of the furnace. The feed above the reaction zone should be porous so as to permit the flow of product gases. Furthermore, the feed should be proportioned and fed in such a manner to allow the feed to descend freely into the furnace without bridging. Feed mixes of too large a particle size or particle size range are generally not used since they can be difficult to procure and cause furnace charging and bridging problems. They may also cause greater electrical resistance. However, too small a particle size in the feed mix can lead to losses by gas entrainment, low bed porosity and mix bridging.
The liquid slag and alloy products are drained from the furnace through a taphole either continuously or intermittently. The slag may separate from the alloy by decantation, skimming or bottom tapping of the receiving ladle. The ferrochrome product is then cast in chills.
Whilst this method of ferrochrome production is most widespread, it does present several disadvantages. Firstly, most or all of the energy requirements of the smelting process are supplied by electricity, which is an expensive form of energy. Secondly, the reductant requirements are met by using coke. Coke is a costly reductant, and is becoming more difficult to obtain as world supplies of coking coals are depleted and increasingly stringent environmental restrictions are placed on the operation of coke oven batteries. Thirdly, the feed particle size limitations preclude the direct use of cheaper fine-sized ore feed.
An alternative technology for the production of ferroalloys (including ferrochrome) which is now emerging is plasma carbothermic smelting reduction. This method has a number of advantages over the submerged arc furnace process:
fine-sized materials are the preferred charge; PA0 the reductant need not be coke-coal fines or coke breeze are suitable; PA0 uniform and consistent charge material properties are not crucial; PA0 slag composition can be selected independently of electrical resistivity, making it possible to operate at a slag composition which minimizes losses of alloy metal to the slag; PA0 process control is much improved, since the process is not as sensitive to charge material properties; and PA0 the plasma furnace operates at lower noise levels. PA0 the difficulties of using finely sized ores directly; PA0 the requirement for expensive coke; PA0 the use of expensive electrical energy for smelting; PA0 the simultaneous control of the states of oxidation of the slag and metal phases; and PA0 the limited use of the chemical energy (reducing potential) and sensible heat of product gases within the smelting vessel. PA0 (a) injecting an alloying metal-containing material and a flux at controlled rates into a bath comprising molten material containing iron or derived from an iron containing material; PA0 (b) injecting an oxygen-containing gas and a carbonaceous material at controlled rates into the bath or into a space above the bath or both; PA0 (c) injecting a gas into the bath to assist reaction gases formed in the bath in creating a transition zone immediately above the bath, the transition zone containing molten material projected from the bath by the gas and the reaction gases; PA0 (d) controlling the rate of injection of the alloying metal-containing material, the flux, the oxygen-containing gas and the carbonaceous material to achieve rapid incorporation of the alloying metal-containing material and the flux into the bath as well as control the oxidation/reduction environment within the bath; PA0 (e) causing the alloying metal to be reduced and report to a metal phase or oxidized and report to a slag phase; and PA0 (f) recovering the phase containing the alloying metal.
However, in spite of these advantages the plasma smelting process still suffers from the serious disadvantage in that all of the smelting energy requirement is supplied in the form of expensive electricity.
In an effort to reduce the cost of manufacturing the ferrochrome alloy, a number of processes have been proposed which avoid supplying the energy for smelting in the form of electricity.
In U.S. Pat. No. 4,565,574 (Nippon Steel Corporation) a process for the production of high chromium alloys by smelting reduction is disclosed. In this process powdered coke and chromium containing ore are pelletized and dried. The pellets are then charged to a rotary kiln, where they are heated and partially reduced. Further coke and limestone flux may be added part-way along the rotary kiln to improve the reduction of the pellets, to preheat the coke and to calcine the limestone.
According to the Nippon Steel patent, the maximum temperature within the rotary kiln is kept above 1400.degree. C. On discharge the prereduced pellets, coke and flux drop from the kiln down a chute into the top of a smelting reduction furnace. This furnace is similar in shape to an ordinary steelmaking converter. The furnace has typically four bottom-blowing tuyeres for oxygen supply, which are protected by propane, whilst the bulk of the oxygen is introduced above the bath through a lance. To maintain control of the temperatures of the slag and metal phases and of the levels of oxidation within these phases, it is necessary to blow oxygen both above and below the bath whilst simultaneously injecting coke into the slag from the top of the smelt reduction vessel.
Smelting of the ore proceeds batchwise, in two stages. Firstly, with a converter temperature of 1580.degree. to 1630.degree. C., preheated, prereduced pellets, coke and flux are charged to the vessel whilst top and bottom blowing with oxygen. A second stage then follows, when no ore or flux is charged, and oxygen additions are progressively reduced, to minimise the chromium content of the slag. However, still more coke must be added to the vessel during this second stage, to control the state of oxidation of the slag and metal phases. The slag and metal are then removed from the vessel.
It is necessary to burn at least 30% of the combustible gases, leaving the bath using an overhead oxygen lance in order to obtain good utilization of the carbonaceous materials and coke used. However, combustion levels above 50% are not desirable due to the quantities of SO.sub.x and NO.sub.x generated.
Furthermore although the specification of U.S. Pat. No. 4,565,574 refers to the need for "hard stirring", the upper limit to stirring intensity is determined by the rate at which the bath lining degrades. At a higher stirring intensity, the stirring of the slag contributes to lining degradation. Stirring intensity is optimized when the temperature of the bath is uniform.
Another process is known (Japanese Patent 58-117852 Sumitomo Metal Industries) where a chrome-containing ore is charged to a bath of molten iron in a top-and-bottom blown converter. Fine chromium ore, fluxes and lump coke are dropped onto the surface of the melt, whilst oxygen is blown softly through a top lance. Coke floating on the slag surface is partially burnt by this oxygen, residual coke being drawn into the slag by agitation induced by oxygen and nitrogen introduced through side blowing nozzles and by bottom blown nitrogen. Circulation produced by the injected gases transfers heat to the slag and metal and allows the coke to reduce the chrome oxide in the slag.
The solid feed is charged to the converter for the duration of the smelting period. A finishing reduction period then follows, during which no solids are charged, and oxygen is introduced only onto the surface of the bath. This finishing reduction lowers the chromium content of the slag and gives a stainless steel grade chromium alloy of 20-32% chromium.
Although this process avoids smelting with electrical energy and can use fine sized ores, it requires lump coke, and each batch requires a charge of molten iron. Furthermore, it is only suitable for making a low chromium alloy. The process does not yield a charge chrome quality ferroalloy.
Another process is also known (Japanese patent 59-107011 Kawasaki) wherein fine chromium-containing ore is optionally prereduced and then fed into a shaft furnace with air or oxygen-enriched air. Lump coke is used as a solid reducing material, and is charged into the shaft furnace from the top. The injected ore melts in front of the tuyeres through which it is injected, and is reduced to the metal as it drips through the coke bed. The furnace hot-reduction zone is increased by injecting coal and an oxygen-containing gas into the shaft furnace through a second row of tuyeres located below the ore-injection tuyeres.
Slag and ferroalloy are tapped from the base of the furnace. Slags have been reported with chromium contents of less than 0.6% and metals containing 8 to 50% chromium have been obtained. Whilst this process also avoids the use of electrical energy for smelting, it is still dependent on the use of lump coke.
Generally speaking, there are major problems in the prior art processes. These include:
Whilst certain of the prior art processes have found particular solutions to some of the above problems, none of the prior art processes discussed above simultaneously solves all of the above problems to the extent achieved by the current invention.
It is known in the prior art to form a molten bath, which contains chiefly iron, iron oxides and slag forming materials, wherein iron oxides can be reduced directly to iron. In one known process, the source of energy is provided by injecting carbonaceous material, carrier gas and protective gas into the bath. At least a part of the fuel undergoes combustion. The reaction gases which are generated agitate the bath causing molten material to be projected from the bath into a transition zone above the level of the bath. Oxygen-containing gas is injected in the form of a jet or jets into the space above the bath. The injected gas combusts with the reaction gases released from the bath. The gases produced impinge on molten material in the transition zone, whereby energy generated by the post-combustion is transferred to the molten material in the transition zone.
It is an object of the present invention to provide a process for the bath smelting of alloying metal-containing material, such as chromium-containing material, to, for example, either a crude stainless steel or a charge chrome quality ferroalloy, which avoids the use of electrical energy for smelting and does not require lump or agglomerated alloying metal-containing materials.
It is a further object of this invention to reduce or eliminate requirements for coke.
It is a further object of this invention to make greater use of the chemical energy and sensible heat of product gases within the smelting vessel.
A further object of this invention is to provide good control of the states of oxidation of the slag and metal phases.